There has been conventionally known a differential gear used in a vehicle such as an automobile, which transmits a driving force (torque) from an engine, through a ring gear, a differential case assembled on the ring gear, a pinion shaft assembled on the differential case, pinion gears assembled on the pinion shaft, and side gears configured to be meshed with the pinion gears. The pinion shaft, the pinion gears, and the side gears, are provided inside the differential case.
As a method for forming such a differential gear, the following forming method has been known from JP2005-504943T.
At first, a differential case (body 1) is composed of a first case body (cage 2) and a second case body (block member 3).
The first case body includes: on one end thereof, a first boss part (tubular part 14) into which a first drive shaft is inserted;
and, on the other end thereof, a flange part (radial ring 18). The second case body includes, on the other end thereof, a second boss part (tubular part 20) into which a second drive shaft is inserted. The first case body (cage 2) is formed by forging.
Then, a pinion shaft (assembling pin 9), pinion gears 6, and side gears (shaft pinions 5) are respectively assembled in the differential case (body 1). Thereafter, a one-end-side outer circumferential surface (cylindrical surface 22) of the second case body (block member 3) is fitted into an other-end-side inner circumferential surface of an other-end-side opening (opening 10) of the first case body (cage 2).
Subsequently, the other-end-side inner circumferential surface of the first case body (cage 2) and the one-end-side outer circumferential surface (cylindrical surface 22) of the second case body (block member 3) are welded to each other, so that the first case body (cage 2) and the second case body (block member 3) are joined to each other.
Finally, a ring gear (crown wheel), which has been formed separately from the differential case (body 1), is brought into contact with the flange part (radial ring 18) of the differential case (body 1), and bolts are inserted into a plurality of bolt holes (holes 19 into which the bolts are inserted) formed in the ring gear (crown wheel) and the flange part (radial ring 18) of the first case body (cage 2), so that the ring gear (crown wheel) and the differential case (body 1) are assembled on each other. In this manner, a differential gear is completed.
In addition, as another method for forming a differential gear, the following forming method has been known from JP7-54961A.
At first, a differential case is composed of a first case body (support member 3), which includes a first boss part into which a first drive shaft (drive shaft 7) is inserted, and a second case body (lid 4), which includes a second boss part into which a second drive shaft (drive shaft 7) opposed to the first drive shaft (drive shaft 7) is inserted. The first case body (support member 3) and the second case body 4 (lid 4) are formed by forging, respectively.
Then, a pinion shaft (differential pin 11), pinion gears (gears 12), and side gears (gears 8 and 9) are respectively assembled in the differential case. Thereafter, a one-end-side outer circumferential surface of the second case body (lid 4) is inserted into an other-end-side inner circumferential surface of an other-end-side opening (opening 22) of the first case body (support member 3).
Subsequently, the other-end-side opening (opening 22) of the first case body (support member 3) and the second case body (lid 4) are welded to each other, so that the first case body (support member 3) and the second case body (lid 4) are joined to each other.
Finally, a ring gear 1, which has been formed separately from the differential case, is fitted into a flange part 2 of the differential case, and the ring gear 1 and the differential case are joined to each other by welding, so that a differential gear is completed.
In the technique for assembling the ring gear and the differential case by means of bolts, which is disclosed by JP2005-504943T, a torque from the ring gear is transmitted only through positions on which the plurality of bolts are fastened (bolt fastening positions). For this reason, the bolt fastening positions, which are located between the ring gear and the differential case, are required to have a further durability; and thus the bolt fastening positions and areas surrounding the same are required to have increased thicknesses. However, this is disadvantageous in that a weight of the differential case is increased. In addition, since the plurality of bolts are necessary, the number of components of the differential gear is increased, which increases costs.
Further, the first case body (cage 2) extending from the first boss part to the flange part is formed by forging. Because of a long axial material-flow-length of the first case body (cage 2) and a long radial material-flow-length thereof, when the first cage body (cage 2) is formed by forging, there occurs a problem in that a heavy forming load has to be applied thereto.
On the other hand, in the forming method of JP7-54961A, since no bolt is used, there can be avoided that a weight of the differential case is increased, which may be caused by increasing thicknesses of the bolt fastening positions and the areas surrounding the same. There can be also avoided that the number of components is increased by a plurality of bolts resulting in cost increase. However, the welded part between the first case body (support member 3) and the second case body (lid 4) and the welded part between the first case body (support member 3) and the ring gear 1 are located in a “torque transmission area” through which a driving force of an engine is transmitted from the ring gear 1 to the side gears (gears 8 and 9). In this case, when a torque is transmitted, there is a possibility that the welding parts might be cracked, because strengths of the welding parts are relatively lower than those of the ring gear 1 and the first case body (support member 3). Namely, there is a problem in that a durability of the differential gear is poor.
In addition, the first case body (support member 3) extending from the first boss part to the other-end-side opening (opening 22) is formed by forging. Because of a long axial length of the first case body (support member 3), when the first case body (support member 3) is formed by forging, a heavy forming load has to be applied thereto.